As is known in the art, there are many reasons for placing sensors, probes, and various kinds of detection instrumentation in subterranean environments. For example, the petroleum industry may use subterranean sensor arrays to study geophysical properties of the deep earth to assist in crude oil exploration and extraction. Construction teams drill boreholes into the earth and install sensor arrays, typically encased in a protective jacket. Once the sensory array is in place, grout may be injected into the borehole cavity to surround the sensor array and to attempt to uniformly couple the sensor array with the surrounding earth. One of the main goals of a drilling operation is to maximize sensor array accuracy and sensitivity by forming a tight acoustical and/or seismic coupling between the sensor array and the surrounding earth.
As is also known in the art, many sensor arrays lack both the strength and ruggedness to survive horizontal directional drilling (HDD) operations. To accommodate these weaker sensor arrays, drilling teams may excavate an open trench, dispose the sensor array at the bottom of the trench, and backfill the trench with grout and/or soil to cover the sensor array. However, open trench excavation may result in voids and air cavities in the surrounding earth, which can significantly impede sensor array performance. Furthermore, open trench excavation often involves moving relatively large amounts of earth, which can be expensive, time-consuming, and is disruptive of the surrounding area.
Open trench excavation may be useful under certain conditions, such as when space is limited or for shallow-depth applications. These installations are often limited to depths of 20 feet or less, and more typically involve depths of ten, five, or even fewer feet. Sensor arrays have limited application at such shallow depths, although construction teams can use them to detect vibrations in manholes and other underground tunnels near the surface.
As is also known in the art, directional boring (so-called “horizontal directional drilling” or HDD) is another technique that industry uses to install sensor arrays and other subterranean devices. Drilling operations often employ HDD where direct-cut open trenching is undesirable or too disruptive. Also, HDD may involve drilling at relatively large depths, such as to install piping under a canal and or to assist in oil exploration.
HDD is a steerable, trenchless method in which teams install devices in a three-stage process including drilling a pilot hole, enlarging the hole, and depositing the device within the larger hole. Drilling teams uses a viscous fluid to help cool the drill bit, remove loosened soil, and to stabilize the hole. To help stabilize the device and to attempt to fill all voids and produce a tight coupling, teams often introduce a grout through one end of a tube or conduit which also contains the installed device. The tube may be retreated back up the opening or pulled through the entire borehole when the team determines (e.g., using sensors) that they have deposited a sufficient amount of grout to stabilize the borehole cavity and/or crevices in the earth.
For example, HDD may be used to install high-power electrical cable which must be uniformly coupled to the surrounding medium, such as the earth, to promote heat transfer from the cables. One suitable material used to protect the cable includes high-density polyethylene (HDPE) plastic. HPDE offers an acoustical impedance similar to that of compact soil or soft rock. HPDE is also rugged, abrasion resistant, waterproof, and relatively inexpensive.